I see that you have really low ISO on all of these. I find that there seems to be a fear of ISO on these new dslr cameras. I have the same sensor in my D7500 and find that I get great images up to about 1600 ISO and then acceptable images up to about 4500 ISO depending on the subject and need for crop.
So a couple of things:
1) SS twice that of the focal length? I believe that is best stated to be: "at least the focal length". So if you have stationary subject, no vibration control and are hand held, then 600 should get you at the long end of that lens.
2) AE Lock on shutter. You might want to turn this off and instead use auto ISO with the maximum setting at 2000. If you lock the exposure, and your subject moves, the camera will not allow the ISO to change for the changing light conditions. Then you can adjust your shutter speed with your back command dial and your aperture with your front command dial to get the two parts of the exposure triangle where you want them and let the camera sensor do the hard part from there.
3)On your playback set up: Playback display options: check "focus point", "highlights" and "shooting data". Then take a test shot. Pull up the image and use the "up down" control to scroll thru the data. There will be a screen that will show "blinkies" if anything is over exposed. THEN you would use the exposure compensation to help in that situation.
My suspicion is that since you were told incorrectly about the SS relative to focal length, that someone has also made you skittish regarding ISO. If we all had perfect light and a budget for super fast mega telephoto lenses (600mm 2.8???) then everyone would have the luxury of shooting at super low ISO.
Here is a link to some high ISO images that I captured recently: www.uglyhedgehog.com/t-571903-1.htm
They have some noise, but most people find them very acceptable. You can hit the link below to my flickr page, then click on each image. If you scroll beneath the image, it will show you basic info on the shot, and if you hit "exif" you will pull up all the info on the shot AND the post processing.
Mostly I shoot birds and BIF: Manual, BBF, single point focus, auto ISO with maximum set at 2000, and -0.3 exposure compensation (because I shoot brighter birds against darker backgrounds) I seldom use a tripod/monopod, but I only have a 300mm lens and it is lightweight. If you have the D500, we share the same sensor, so the exposure results should be very similar.
Let me know if I can be of more help.